Allow users to activate an all-in-one 'scroll then DL' action

[bajuwa]

Probably do this via something like:

• Add new button for 'Scroll + DL'
• When triggered, call the existing auto-scroll function, but also introduce a new timer that runs on a longer interval for 'check for scroll finished'
• 'Check for scroll finished' should store a 'previous window y position' value and compare it to the current one every interval. If they match, cancel all timers and trigger download. If they don't match, override the 'previous' with the 'current' and wait for next interval